1. Field of the Invention
The present invention relates to a method and a device for operating an internal combustion engine, in particular a method for operating an internal combustion engine in the transition from a partial engine operation to a full engine operation.
2. Description of Related Art
In internal combustion engines having direct injection, in particular spark ignition engines, in order to reduce the drive torque which is provided, the internal combustion engine may be operated in an operating mode in which a portion of the cylinders are switched off, i.e., operated in such a way that no combustion occurs therein, and therefore no drive torque is generated in these cylinders. Such an operating mode is referred to as “partial engine operation.” The operating mode in which one-half of the cylinders are switched off is referred to as “half-engine operation.”
When the internal combustion engine is operated in partial engine operation, for the affected switched-off cylinders the intake valves and exhaust valves for respectively supplying and discharging gases are closed, and the injection for these cylinders is suspended. The affected cylinders are switched off during the transition to partial engine operation, immediately after an air-fuel mixture has been combusted in the affected cylinder. After combustion the exhaust valve remains closed, so that the combustion exhaust gases present in the cylinder remain there. The internal combustion engine is driven by the cylinders which are active, i.e., not switched off, and provides a corresponding drive torque. This causes the pistons in the switched-off cylinders to be moved, so that the combustion exhaust gas present therein from the preceding combustion is successively compressed and expanded.
A switch is made between full engine operation, in which all cylinders are active, and partial engine operation, in which only a portion of the cylinders are active, as a function of an operating point, such as the load and/or the rotational speed, for example, and/or as a function of an external requirement, such as a driver input, which may be specified by a gas pedal position.
In direct fuel injection, fuel is injected directly into the combustion chamber with the aid of injectors. Even when the injectors are closed, leakage of fuel into the cylinder cannot be ruled out. For example, for conventional injectors leakage of up 2.5 mm3 per minute is possible, and is not uncommon. When the internal combustion engine is operated in partial engine operation, fuel may enter the combustion chambers of the switched-off cylinders as the result of leakage into the switched-off cylinders. The enclosed combustion exhaust gas is thus enriched with uncombusted fuel. Since the partial engine operation may last for several minutes on average, the quantity of fuel which passes into the combustion chambers of the switched-off cylinders is not negligible. As soon as the internal combustion engine is switched from partial engine operation back to full engine operation, the combustion, exhaust gas which is enriched with fuel is discharged into the exhaust gas system of the internal combustion engine.
However, as the result of regulatory requirements, emission limits have been established for hydrocarbons emitted to the environment. These requirements apply for all operating modes, i.e., for full engine operation, partial engine operation, and also during the switchover phase between the two operating modes.
For internal combustion engines having direct fuel injection, a catalytic converter is usually used for exhaust gas enhancement for reducing the emissions of nitrogen oxides and carbon monoxide. Such a catalytic converter is able to store or release oxygen up to a certain degree. For a lean mixture, which results in combustion exhaust gases in which an air fraction is present, oxygen is stored in the catalytic converter. For a rich mixture, which results in combustion exhaust gas in which uncombusted hydrocarbon (HC) fractions are present, the catalytic converter releases the oxygen and oxidizes the uncombusted hydrocarbon fractions in the combustion exhaust gas.